Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) {
            return null;
        }
        List<String> ans = new ArrayList<>();
        Deque<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            if (node != null) {
                ans.add(node.val + "");
                q.offer(node.left);
                q.offer(node.right);
            } else {
                ans.add("#");
            }
        }
        return String.join(",", ans);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null) {
            return null;
        }
        String[] vals = data.split(",");
        int i = 0;
        TreeNode root = new TreeNode(Integer.valueOf(vals[i++]));
        Deque<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            if (!"#".equals(vals[i])) {
                node.left = new TreeNode(Integer.valueOf(vals[i]));
                q.offer(node.left);
            }
            ++i;
            if (!"#".equals(vals[i])) {
                node.right = new TreeNode(Integer.valueOf(vals[i]));
                q.offer(node.right);
            }
            ++i;
        }
        return root;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));